如何计算SQL中每个值的表行？

Question

I am new here so if its not fixed topic please fix me. I have this table:

id | uid | month | year
-----------------------
1 | 5 | 12 | 2018
2 | 5 | 12 | 2018
3 | 9 | 12 | 2018
4 | 3 | 01 | 2019

I want to count how much times for each uid the month and year values equal to - month=12 AND year=2018

for example I want to get response like that:

uid=5 - count = 2
uid=9 - count = 1

How its possible?

Answer 1

With group by uid :

select uid, count(*) as counter
from table
where month = 12 and year = 2018
group by uid 

Answer 2

You can use conditional aggregation:

select uid,
       sum(case when year = 2018 and month = 12 then 1 else 0 end) as cnt
from t
group by uid;

This will include uid s with a count of 0. If you just want uids where the count is greater than 0, use a where clause:

select uid, count(*)
from t
where year = 2018 and month = 12
group by uid;

Answer 3

选择uid，从year = 2018和month = 12按uid分组的表中选择count（*）